osmo-bts-trx: Increase a maximum allowed MS power reduction step from 2dB to 4dB.
osmo-bts-trx: Increase a maximum allowed MS power reduction step from 2dB to 4dB.

We tend to start MS with high power to make sure distant phones get good QoS,
but this also means that we need to reduce their power rather quickly. OTOH
we can't make this step too high because this may lead to power output
oscillation. From my (manual, limited) testing 4dB looks like a reasonable
compromise.

Change-Id: I58785513e5739474b881ed7f2a312ecc690e7e60

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts